The present invention relates generally to channel estimation in communication systems and in particular to channel estimation techniques for wireless communication systems using multiple subcarriers, including systems compliant with the IEEE 802.11a standard.
In a typical wireless communication system in use today, a transmitter and a receiver communicate in accordance with a protocol such as the IEEE 802.11a standard. The standard specifies a keying scheme that generally involves grouping. Bits of data to be transmitted are grouped and each group of bits is mapped to a symbol (a particular combination of phase and amplitude) in a signaling constellation. A number of constellations (or keying schemes) are known in the art, including binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM) constellations. The transmitter includes an encoder that transforms an input data stream into a sequence of symbols. The transmitter may also insert additional information-carrying symbols into the input data stream. For example, protocols such as IEEE 802.11a specify a “preamble” to precede transmitted data. The preamble usually includes parameter values related to various features of the signal (e.g., data rate, keying scheme, etc.) as well as a known sequence of bits or symbols (referred to herein as a “training sequence”) that the receiver can use for calibration. The symbols are modulated using an appropriate protocol (e.g., OFDM (orthogonal frequency division multiplexing) in the case of IEEE 802.11a) into a form suitable for transmission over a channel. The receiver receives the signal, demodulates it, and generates an output data stream that ideally is identical to the input data stream.
In practice, the output data stream has a non-zero bit error rate that depends on various factors. For example, channel properties such as fading and delay, as well as noise, are known to affect the received signal. Receivers in use today generally include circuitry for estimating and compensating for channel and noise effects.
In typical communication protocols, channel effects are generally estimated by measuring the receiver's response to a known sequence of input symbols that is inserted into the input data stream. For instance, FIG. 1 illustrates a transmission in a system complying with the IEEE 802.11a standard. Each transmission begins with a 20 microsecond (μs) preamble having short training symbols S, a guard interval LG, long training symbols L, and a signal field (4 microseconds). The preamble is followed by data. The first eight microseconds of the preamble comprise ten identical short training symbols S (0.8 μs each) that are used for packet detection, automatic gain control and coarse frequency estimation. The second eight microseconds comprise two identical long training symbols L (3.2 μs each), preceded by a guard interval LG that is the same pattern as the last half (1.6 microseconds) of the long training symbol L. The long training symbols can be used for channel estimation, timing, and fine frequency estimation. The signal field provides information about the data, such as the length of the packet, encoding scheme (or data rate), and the like.
FIG. 2 shows a long training sequence L1 that is used to generate the signal representing the long training symbol in a conventional 802.11a preamble. This sequence represents values (phase shifts) used for each of the 64 subcarriers. As specified in the standard, the subcarriers span a 20 MHz channel and are spaced apart by 312.5 kHz. By convention, used here, the first value in the sequence L1 is the value for the DC subcarrier (index k=0), followed by the value for the 1×312.5 kHz subcarrier, then the value for the 2×312.5=625 kHz subcarrier, etc., up to the 32nd value for the 31×312.5 kHz=9687.5 kHz subcarrier. The 33rd value corresponds to the −10 MHz subcarrier, followed by the −(10 MHz-312.5 kHz) subcarrier, and so on, with the 64th (index k=63) value corresponding to the −312.5 kHz subcarrier. As can be seen from FIG. 2, the DC value and the 28th through 38th values (corresponding to subcarriers near the edges of the 20 MHz channel that are not used to transmit data) are zero.
The output of the transmitter is a long training symbol at a sample rate of 64 samples/symbol. The samples are obtained by taking a 64-point IFFT (inverse Fast Fourier transform) of the long training sequence L1. As used herein, a sequence in the frequency domain is expressed with uppercase letters (e.g., L[k]), while the corresponding time sequence is expressed with lowercase letters (e.g., l[i]). The transformed samples l[i] are modulated according to the RF channel frequency, converted to analog, and transmitted.
The receiver detects the RF signal, converts it to digital (at standard 20 MHz sampling rate) and demodulates it. Thus, the receiver generates 64 samples during each repetition of the long training symbol, for a total of 128 training samples. These training samples r[i] (where i=0, 1, . . . , 127) are used to compute channel response according to various conventional algorithms. For example, one such algorithm involves averaging the corresponding samples from the two repetitions of the long training symbol: r[i]=(r[i]+r[i+64])/2, i=0, 1, . . . , 63.  (Eq. 1)A Fast Fourier Transform (FFT) is performed on the averaged time-domain samples r[i] to obtain 64 subcarrier components S[k], k=0, 1, . . . , 63, representing the channel amplitude and phase for each subcarrier, phase shifted according to the components of the training sequence L1. The phase shift due to the training sequence is removed by multiplying each subcarrier coefficient S[k] by the corresponding training sequence element L1[k], and an IFFT is performed on the result to obtain a complex-valued channel impulse response ĥ[i] for each subcarrier.
Recently, there has been interest in developing multiple input, multiple output (MIMO) wireless systems to provide increased data rates. Such systems generally include a transmitter system having a number (Mt) of transmit antennas (transmitters) communicating with a receiver system having a number (Mr) of receive antennas (receivers), where Mr and Mt may or may not be equal. In a MIMO communication system, each of the Mt transmit antennas transmits, at substantially the same time, a symbol representing a different group of bits from the input data stream. If each symbol represents B bits, the number of bits transmitted per symbol period is B*Mt. Thus, MIMO systems are considered desirable as a way to increase bandwidth of the communication system. For example, a conventional SISO system compliant with IEEE 802.11a can transmit up to 54 Mbps (megabits per second). In a MIMO system with Mt transmit antennas, the data rate can be increased to Mt*54 Mbps.
MIMO systems introduce additional complexity to the channel estimation that the receiver needs to perform. Specifically, each receive antenna receives a signal that is a combination of signals from the transmit antennas, modified by the channel properties and noise. In effect, there is a channel from each transmitter to each receiver, and separate channel estimates must be made for each such channel. Thus, the per-carrier channel estimate h[i] becomes an Mr×Mt matrix [i].
Various techniques for modifying the IEEE 802.11a preamble for use in MIMO systems have been suggested. These techniques generally involve changing or repeating the preamble to allow separate calibration for each transmit antenna. Some of these techniques, such as having each transmit antenna transmit the training sequence while other antennas transmit nothing, can lengthen the training period and reduce the data rate.
It would be desirable to provide improved channel estimation techniques for use in MIMO-extended 802.11a and other wireless communication systems.